Vacuum deposited metal thin film coatings have found widespread application as transparent resistance heaters for deicing and demisting transparent enclosures such as windshields. Typically, such enclosures comprise a thin transparent plastic carrier film or substrate upon which has been deposited a thin conductive metallic film which can be powered by busbars normally attached at the edge portions thereof. Ultimately, the metallized substrate is laminated between rigid, transparent plies as of glass in order to form a safety glass-type construction.
In order that upon energizing power be dissipated in an optimal manner for the particular deicing function of the ultimate laminated transparency, and to preserve the optical properties required in windshield and like employments, resistivity distribution of the deposited metallic film must be closely controlled. In the past, local resistivity has been determined by measuring the overall (bus to bus) resistance of the final part and then normalizing the value determined to a square area configuration. This requires physical contact to a finished part already equipped with busbars. However, it is frequently desirable to determine resistivity of a metallized substrate film prior to the application thereto of busbars or other means of electrical attachment. In U.S. Pat. No. 3,086,889 to Strong, it is proposed that electrical resistance of a continuously deposited metallic coating be monitored by balancing a bridge circuit against resistance measured between electrical contacts which engage the metallic coated side of a dielectric sheet material. Unfortunately, where the metallic film whose resistance is to be measured is so thin as is the case where transparency is required, then the fragility of the metallic coating is highly susceptible to disruption by such contact methods of measurement. When the metallized film is subsequently energized, disruptions in the fragile coating give rise to localized regions of high current density which can lead to failure. Accordingly, a need has existed for a means of non-destructively describing the local resistivity of vacuum-deposited thin metal films.
In U.S. Pat. No. 3,397,672 to Dykeman, et al., it is proposed at column 3, lines 2-5, to measure the thickness of vapor-deposited aluminum by detection of radiation emanant from the coating upon application of x-rays. The equipment required for x-ray generation is generally highly expensive and to the economic disadvantages of that proposal must be added the danger to operators and maintenance personnel inevitably inhering in x-ray operation.
In determining what form of radiation will be employed and detected in monitoring resistivity of vacuum-deposited metals, however, more is involved than straightforward considerations of economy and safety. There is required a wavelength interval which when detected provides a relatively steep and linear curve of transmission or reflectance against resistivity. Moreover, where transmission is to be measured, a spectral region must be chosen wherein transmission of the bare substrate is high, i.e., a window.